A rim structure of a wheel for an off-road type large vehicle such as those used in mines is typically composed of multiple pieces to make it easy to mount a large-weight tire.
A known multi-piece rim structure 1 shown in FIG. 6 includes five pieces, i.e., a rim base 10, a bead seat ring 20 (ring member), a lock ring 30 and a pair of side rings 40, 50. “Axial direction”, “radial direction” and “circumferential direction” used in this specification respectively refer to an axial direction, a radial direction and a circumferential direction of the multi-piece rim structure. “Outside in the axial direction” refers to a side away from a central position of a tire set in the multi-piece rim structure in the axial direction. “Inside in the axial direction” refers to a side closer to the central position of the tire in the axial direction.
The rim base 10 is made by welding three short cylindrical members one by one arranged in the axial direction (width direction). Each of the short cylindrical members is made by rounding a strip of rolled steel plate into a cylindrical shape and welding opposite end surfaces thereof.
The rim base 10 includes a gutter band portion 11 on one side thereof in the axial direction and a back flange portion 12 on the other side thereof.
The bead seat ring 20 is disposed on an outside of the gutter band portion 11 of the rim base 10 in the radial direction. One of the side rings 40 is disposed on an outside of the bead seat ring 20 in the radial direction. The side ring 40 is caught by an annular raised flange portion 21 formed in a peripheral edge of the bead seat ring 20 located outside in the axial direction. The other side ring 50 is caught by the back flange portion 12 of the rim base 10.
An outer peripheral surface of the bead seat ring 20 is provided as a bead seat portion 22 having a width W. An outer peripheral surface of a portion of the rim base 10 adjacent to the side ring 50 is provided as a bead seat portion 13 having a width W. The bead seat portions 22, 13 are arranged to have a pair of bead portions of a tire (not shown) placed thereon. An annular load applying surface 23 having a tapered configuration is formed in an inner periphery of an edge portion of the bead seat ring 20 located outside in the axial direction.
To mount a tire to the multi-piece rim structure 1 described above, the side ring 50, the tire, the side ring 40 and the bead seat ring 20 are moved in this order in the axial direction from the gutter band portion 11 toward the back flange portion 12 to be mounted on the rim base 10, and finally, the lock ring 30 is received in the rim base 10.
Prior to the mounting of the bead seat ring 20, a seal ring 60 that is an O-ring is fitted into a seal ring groove 16 (to be described later) of the gutter band portion 11. The seal ring 60 is provided for sealing between the bead seat ring 20 and the gutter band portion 11.
A multi-piece rim structure 1′ shown in FIG. 7 is also well-known. The multi-piece rim structure 1′ includes three pieces, i.e., a rim base 10′, a ring member 5 and a lock ring 30. The rim base 10′ integrally includes a side ring portion 50′. The side ring portion 50′ corresponds to the side ring 50 and the back flange portion 12 of the multi-piece rim structure 1 shown in FIG. 6. The ring member 5 integrally includes a bead seat ring portion 20′ and a side ring portion 40′. The bead seat ring portion 20′ and the side ring portion 40′ respectively correspond to the bead seat ring 20 and the side ring 40 of the multi-piece rim structure 1 shown in FIG. 6. Other features are similar to those shown in FIG. 6, and therefore, are designated by the same reference numerals and description thereof will be omitted.
Rim structures 1A, 1B used in a dual-wheel type vehicle shown in FIG. 8 are also well-known. The rim structure 1A that holds an inner tire Ta closer to the vehicle has similar features to those in FIG. 6. In the rim structure 1B that holds an outer tire Tb farther from the vehicle, opposite end portions of a rim base 10″ in the axial direction are provided as gutter band portions 11. A bead seat ring 20, a lock ring 30 and a side ring 40 are mounted on each of the gutter band portions 11.
The gutter band portion 11 and the lock ring 30 of the rim structures 1, 1′, 1A, 1B will be described hereinafter particularly referring to FIG. 9. A lock ring groove 15 and a seal ring groove 16 are formed in an outer peripheral surface of the gutter band portion 11. The seal ring groove 16 is disposed inside in the axial direction with respect to the lock ring groove 15. The lock ring groove 15 has a concavely curved cross-sectional contour. A first receiving surface 17 that is a circular cylindrical surface is formed inside in the axial direction with respect to the lock ring groove 15, i.e. between the lock ring groove 15 and the seal ring groove 16. A second receiving surface 18 that is a circular cylindrical surface is formed outside in the axial direction with respect to the lock ring groove 15, i.e. between the lock ring groove 15 and an outer side edge of the gutter band portion 11.
The lock ring 30 is disposed between the gutter band portion 11 of the rim base 10 and the bead seat ring 20 (or the ring member 5). The lock ring 30 has an annular configuration that is cut at one point. A load receiving surface 31 having a tapered annular configuration is formed in an outer periphery of an edge portion of the lock ring 30 located inside in the axial direction.
The lock ring 30 includes an annular ridge 35 at an intermediate position of an inner periphery thereof in the axial direction. The ridge 35 continues in the circumferential direction. The lock ring 30 further includes a first abutment surface 37 that is a circular cylindrical surface located inside in the axial direction with respect to the ridge 35. The lock ring 30 further includes a second abutment surface 38 that is a circular cylindrical surface located outside in the axial direction. The ridge 35 has a convexly curved cross-sectional contour.
In a state where the lock ring 30 is mounted in the gutter band portion 11, the ridge 35 is received in the lock ring groove 15 of the gutter band portion 11. The load receiving surface 31 is surface-contacted with the load applying surface 23 of the bead seat ring 20 (or the ring member 5). The first abutment surface 37 is surface-contacted with the first receiving surface 17 of the gutter band portion 11. The second abutment surface 38 is surface-contacted with the second receiving surface 18.
In the multi-piece rim structures 1, 1′, 1A, 1B having the tire mounted thereon, a load in the radial direction is applied to the bead seat ring 20 (or the bead seat ring portion 20′ of the ring member 5) from one of the bead portions of the tire. Moreover, a load in the axial direction is applied to the bead seat ring 20 (or the bead seat ring portion 20′ of the ring member 5) from the one of the bead portions via the side ring 40 (or the side ring portion 40′). Most of the load in the radial direction and the load in the axial direction applied to the bead seat ring 20 (or the ring member 5) is transmitted to the lock ring 30 via the load applying surface 23 and the load receiving surface 31 and transmitted to the gutter band portion 11 from the lock ring 30.
A load transmission path from the lock ring 30 to the gutter band portion 11 will be described in detail. The load in the radial direction is transmitted via a surface contact region R1 in which the first abutment surface 37 and the first receiving surface 17 are contacted and a surface contact region R2 in which the second abutment surface 38 and the second receiving surface 18 are contacted. The load in the axial direction is transmitted via a surface contact region R3 in which a portion of an outer surface of the ridge 35 of the lock ring 30 located outside in the axial direction and a portion of an inner surface of the lock ring groove 15 located outside in the axial direction are contacted.
In the surface contact regions R1, R2, R3, minute reciprocating sliding may occur between the gutter band portion 11 and the lock ring 30 and repeated load may be applied accompanying the running of the vehicle, which may lead to thinning caused by abrasion. Particularly in the surface contact region R3, it is known that cracks 100 may be generated due to fretting fatigue. If such thinning and cracks are left unattended, parts may come off while running, which may make the vehicle immovable.
Thinning due to corrosion may also occur in the multi-piece rim structures 1, 1′, 1A, 1B mentioned above. Specifically, the ridge 35 of the lock ring 30 is received in the lock ring groove 15 of the gutter band portion 11 with play in the radial direction and the axial direction. Since the lock ring 30 receives load outward in the axial direction from the bead portions of the tire, a gap 70 may be formed between an area from a portion of the outer surface of the ridge 35 located inside in the axial direction to a top portion of the outer surface of the ridge 35 and an area from a portion of the inner surface of the lock ring groove 15 located inside in the axial direction to a bottom portion of the inner surface of the lock ring groove 15. Areas of the outer surface of the ridge 35 and the inner surface of the lock ring groove 15 spacedly opposed to each other with the gap 70 interposed therebetween is indicated by reference numeral R0. It should be noted that the lock ring 30 is temporarily moved inward in the axial direction when the lock ring 30 is removed or attached for exchanging tire or when air pressure of the tire is reduced or when the air pressure is increased, and the gap 70 disappears in the region R0. In this condition, the ridge 35 of the lock ring 30 is surface contacted with the portion of the inner surface of the lock ring groove 15 located inside in the axial direction.
Water may pool in the gap 70 if water enters the lock ring groove 15 from a cut portion of the lock ring 30 or if moisture existing in the gap 70 condensates. Therefore, the regions R0, R1, R2, R3 may be corroding environment, in which corrosion thinning may easily occur in the surface contact regions R1, R3.
Particularly, in the surface contact region R3, the fretting fatigue may be accelerated by corrosion and abrasion because stress concentration is significant in this region. Therefore, growth of the cracks 100 may be accelerated in this region.
To cope with this problem, as disclosed in Patent Document 1 to be described later, the applicant suggests providing fretting resistance and corrosion resistance to the multi-piece rim structure by forming painted film layers of normal temperature drying paint on the inner surface of the lock ring groove of the gutter band portion and the first and second receiving surfaces adjacent to the lock ring groove and forming coated layers of normal temperature drying lubricant on the painted film layers.
As disclosed in Patent Document 2 to be described later, the applicant also suggests forming hardened layers on the inner surface of the lock ring groove of the gutter band portion and the first and second receiving surfaces by ion nitriding treatment.